Immunomodulatory effects of anti-CD4 antibody in host resistance against infections and tumors in human CD4 transgenic mice

Anti-CD4 antibodies, which cause CD4(+) T-cell depletion, have been shown to increase susceptibility to infections in mice. Thus, development of anti-CD4 antibodies for clinical use raises potential concerns about suppression of host defense mechanisms against pathogens and tumors. The anti-human CD4 antibody keliximab, which binds only human and chimpanzee CD4, has been evaluated in host defense models using murine CD4 knockout-human CD4 transgenic (HuCD4/Tg) mice. In these mice, depletion of CD4(+) T cells by keliximab was associated with inhibition of anti-Pneumocystis carinii and anti-Candida albicans antibody responses and rendered HuCD4/Tg mice susceptible to P. carinii, a CD4-dependent pathogen, but did not compromise host defense against C. albicans infection. Treatment of HuCD4/Tg mice with corticosteroids impaired host immune responses and decreased survival for both infections. Resistance to experimental B16 melanoma metastases was not affected by treatment with keliximab, in contrast to an increase in tumor colonization caused by anti-T cell Thy1.2 and anti-asialo GM-1 antibodies. These data suggest an immunomodulatory rather than an overt immunosuppressive activity of keliximab. This was further demonstrated by the differential effect of keliximab on type 1 and type 2 cytokine expression in splenocytes stimulated ex vivo. Keliximab caused an initial up-regulation of interleukin-2 (IL-2) and gamma interferon, followed by transient down-regulation of IL-4 and IL-10. Taken together, the effects of keliximab in HuCD4/Tg mice suggest that in addition to depleting circulating CD4(+) T lymphocytes, keliximab has the capability of modulating the function of the remaining cells without causing general immunosuppression. Therefore, keliximab therapy may be beneficial in controlling certain autoimmune diseases.

Comparative pharmacodynamics of keliximab and clenoliximab in transgenic mice bearing human CD4

Keliximab and clenoliximab are monkey/human chimeric CD4 monoclonal antibodies (mAbs) of the IgG1 and IgG4 isotypes, respectively. The pharmacokinetics (PK) and pharmacodynamics (PD) of these mAbs were evaluated in transgenic mice bearing human CD4 molecules on their T cells after a single i.v. administration at three dose levels (5-125 mg/kg). The PK of keliximab and clenoliximab were similar, dose-dependent, and adequately described by a two-compartment model with saturable elimination from both compartments. The enumeration of circulating CD4(+) T cells and density of CD4 on their surface were determined as the PD effects. An indirect response model was proposed to characterize the PD effects. With the increase in mAb dose, the maximum intensity (R(max)) of PD effects was increased, and the time to reach R(max) shifted to later times. At all three dose levels, keliximab caused a relatively rapid decline in the number of circulating CD4(+) T cells, which then recovered gradually. In contrast, clenoliximab at the lowest dose (5 mg/kg) did not produce a significant effect on CD4(+) T cell counts compared with the placebo group. At high doses, clenoliximab caused a significant decrease in the number of CD4(+) T cells. Keliximab appeared to be more potent and efficient in depleting CD4(+) T cells. Both mAbs produced similar down-modulation of CD4 at corresponding dose levels. The findings of this study are consistent with the results of a recent clinical trial that emphasize the importance of this transgenic mouse model for evaluating PK/PD to support clinical development of anti-human CD4 mAbs.

Flow cytometry in the preclinical development of biopharmaceuticals

Novel biomarkers are often required in the preclinical development of biopharmaceuticals in order to characterize pharmacologic and toxicologic effects and to establish pharmacodynamic and pharmacokinetic relationships. Flow cytometry is uniquely suited for measurement of these biomarkers. Large numbers of single cells in a heterogeneous population can be rapidly identified and characterized with high accuracy and reproducibility. Cells are not damaged by the detection system and can be subsequently sorted for further morphologic or functional analysis. The availability of clinical instruments and a wide range of fluorescent probes have made this technology applicable for use in toxicologic clinical pathology. Flow cytometry has played an integral role in the development of a monoclonal antibody to human CD4 (keliximab, IDEC-CE9.1, SB 210396). Lymphocyte subset analysis and assays for expression, coating, and modulation of human CD4 were used for sequential assessment of the pharmacologic activity of keliximab in transgenic mice expressing human CD4.